With the increase of the recording density of magnetic recording, the surface of the magnetic recording layer must be smoother, which results in increasing the coefficient of friction between the magnetic layer while running and each part of a recording and reproducing apparatus, so that smooth running of a magnetic recording medium is hindered and the magnetic layer tends to be damaged. Furthermore, with the popularization of video tape recorders (VTR), personal computers, word processors, etc., as general public devices, the condition for using magnetic recording media, in particular, a temperature and humidity condition becomes more varied.
Moreover, since rotary type magnetic recording media such as floppy disks, etc., for personal computers, word processors, etc., are used for a system wherein an on-off action of a magnetic head is repeatedly applied many times onto the magnetic layer, the running durability of the magnetic recording medium becomes even more important during continuous operation over a prolonged time period at high temperature or under a temperature cycle between high temperature and low temperature.
For solving the aforesaid problems, a method has been proposed of incorporating fatty acid esters into the magnetic layers as described in JP-A-50-22603, JP-A-50-153905, and JP-A-55-139637 (the term "JP-A" as used herein means an "unexamined Japanese patent publication") and JP-B-39-28367, JP-B-41-18065, and JP-B-47-12950 (the term "JP-B" as used herein means an "examined published Japanese patent application"), a method of incorporating silicone compounds in the magnetic layers as described in U.S. Pat. No. 2,654,681, a method of incorporating fatty acids and hydrocarbons in the magnetic layers, etc.
However, in these conventional techniques, there are problems that the aforesaid additives are liable to vaporize from the surface of the magnetic layer under a high temperature condition and, on the other hand, the additives precipitates on the surface of the magnetic layer under a low temperature condition to cause clogging of a magnetic head and the increase of drop out. Also, if the amounts of the additives are increased for improving the effect thereof, they plasticize a binder resin in the magnetic layer to weaken the film strength of the magnetic layer, whereby the durability of the magnetic recording medium is rather reduced.
For solving the aforesaid problems, a method of incorporating a fatty acid ester having a high molecular weight and having a branched hydrocarbon group or an unsaturated hydrocarbon group so that the additive does not readily vaporize from the magnetic layer under high temperature and does not deposit on the surface of the magnetic layer under low temperature, is disclosed in JP-A-58-160425, JP-A-58-218038, JP-A-60-205827, and JP-A-62-125529. However, the esters disclosed in the aforesaid patent publications are in liquid states at room temperature and are compatible with the binder resin of the magnetic layer to plasticize the binder resin, which results in reducing the film strength of the magnetic layer. Thus, a sufficient effect has not yet been obtained by the aforesaid techniques.
Also, as another method for solving the aforesaid problems, a method of using a fatty acid ester having a low melting point in combination with a fatty acid ester having a high melting point is disclosed in JP-A-61-29437 but a sufficient effect is not always obtained in the method.
Furthermore, as still another method, a method of using liquid paraffin and a phthalic acid ester in combination is disclosed in JP B-57-51170, JP-B-60-49972, and JP-B-60-49973. However, although the method has an advantage that oozing out of the liquid paraffin onto the surface of the magnetic layer is properly restrained due to the action of the phthalic acid ester, the phthalic acid ester is liable to vaporize from the magnetic layer at high temperature since the alcohol moiety of the phthalic acid ester is not large and hence there is yet a problem in the running durability of the magnetic recording medium.